What you are looking for is readily available. Clevo supplies lappies chassis capable of such an upgrade or original build, but you have to have a compatible MoBo and chassishttp://forum.notebookreview.com/sager-clevo/91510-clevo-guide-v2-0-faq-reseller-info.html
CLEVO is a large Taiwanese computer company specializing in laptops. While the Clevo brand name is perhaps not widely known, their products are re-branded and sold by known boutique brand OEMs (Original Equipment Manufacturers)… notably Sager, VoodooPC, Falcon Northwest, Eurocom, etc. They are also considered (by whoever knows about notebooks) to design and manufacturer the best of the best notebooks in terms of superior build quality and innovative designs.
The Clevo 370EM for example comes with the following GFX choices:
AMD® Radeon™ HD 7970M 2GB GDDR5 - ETA April 29, 2013 $0.00
Dual NVIDIA® GeForce™ GTX 670MX's Total 6GB GDDR5 in SLI $150.00
NVIDIA® GeForce™ GTX 680M 4GB GDDR5 Memory $200.00
Dual AMD® Radeon™ HD 7970M's total 4GB GDDR5 in CrossFireX™ - ETA April 29, 2013 $350.00
Dual NVIDIA® GeForce™ GTX 680M's total 8GB GDDR5 in SLI! $850.00
NVIDIA® Quadro™ K5000M Graphics with 4GB GDDR5 Video Memory $1,800.00http://www.lpc-digital.com/sager-np9370-features.html
If you had one with the single 680M and wanted to upgrade, it could certainly be done.... whether you could find someone willing to sell you the parts is another story. I have had my GPU upgraded in a Clevo laptop tho it was done during a warranty replacement, I just paid for the difference in cost between the one that died and the newer one.